An insulating gate bipolar transistor (Hereinafter referred to as “IGBT”) is known as a switching element. The IGBT switches on or off a current flowing in a thickness direction of a semiconductor chip composing the IGBT. The semiconductor chip is required to be thinner in order to reduce the power loss when the switching element is in an ON state.
In a conventional method of manufacturing such a kind of switching element, a plurality of semiconductor elements are formed on a semiconductor wafer, and then a recess is formed on the back side of the semiconductor wafer in order to make a portion of the semiconductor wafer thinner. By forming the recess, a rim portion is formed at the peripheral area of the semiconductor wafer. Subsequently, a dicing tape is adhered to the back surface of the semiconductor wafer. Then, the semiconductor wafer is diced with a blade so that the semiconductor wafer is divided into a plurality of semiconductor chips.
When the dicing tape is adhered to the back surface of the semiconductor wafer, a connection portion, i.e. a corner portion, between the recess portion and the rim portion can be prevented from adhering to a base portion of the rim portion so that floating or separating of the dicing tape may occur partially. The floating or separating is caused due to existence of a step which is formed by forming the recess.
In addition, when the semiconductor wafer is set on a vacuum chuck stage before dicing, a clearance needs to be provided between the recess portion and the vacuum chuck stage, in consideration of likelihood of variation in the shape of the formed recess portion. The portion of the semiconductor wafer facing the clearance is not supported by the vacuum chuck stage so that chip cracking or chipping of the semiconductor wafer is likely to occur during dicing.
In Japanese Patent Application publication No. 2007-134454, a method of dicing a semiconductor wafer using a laser beam is disclosed.
According to the method, a plurality of elements is formed on a main surface of the semiconductor wafer. Slits are formed on a back surface of the semiconductor wafer by a mechanical or chemical process. The slits extend along dicing lines or chip dividing lines. Laser beam is radiated to the slits from below in order to produce a reformed layer, i.e. a property modified layer, in a portion of the semiconductor wafer deeper than the slits. The semiconductor wafer is divided starting from the position of the reformed layer as a starting point of cleavage. The divided semiconductor wafer is ground and removed up to a predetermined depth from the back surface.
According to the method, the laser beam is radiated to the slits along the dicing lines or the chip dividing lines in order to produce the reformed layer. As a result, occurrence of minute cracks can be prevented.
However, the patent publication does not show a method for preventing chip cracking or chipping of a semiconductor wafer, which is utilized when a recess portion is formed on a back surface of the semiconductor wafer.